Resonance MEMS mirror control system

ABSTRACT

The present disclosure provides a system and method for controlling operation of a resonance MEMS mirror. The system and method includes activating either an in-plane or staggered MEMS mirror via sets of activation pulses applied to the MEMS mirror, detecting current at the MEMS mirror, generating a window for detecting a change in a direction of the current at the MEMS mirror, and terminating the window and the activation pulse if a change in the current direction is detected during the window. In some embodiments, two sets of activation pulses are applied to the MEMS mirror.

FIELD OF THE INVENTION

The present disclosure generally relates to Micro Electro MechanicalSystems (“MEMS”) mirrors and, more particularly, to a control system forresonance MEMS mirrors.

BACKGROUND

Certain devices such as wafer defect scanners, laser printers, documentscanners, projectors and the like make use of a narrow collimated laserbeam that usually scans across a flat surface along a straight linepath. A typical optical scanning system for this purpose employs atilting flat mirror to deflect the beam. The tilting micro-mirror servesas a central element in many Micro Electro Mechanical Systems (“MEMS”)devices and/or Micro Opto Electro Mechanical Systems (“MOEMS”) devices.For the sake of convenience, these devices (i.e. MEMS and/or MOEMS) willbe referred to herein as “MEMS” devices.

Many of these MEMS devices comprise two types of electro-static mirrors:in-plane mirrors and staggered mirrors. In-plane mirrors are usuallydriven at a resonance frequency. The stator and the rotor of in-planemirrors are fabricated on a single layer, and the mirror's driving pulseis usually of a rectangular type signal. Staggered mirrors are typicallycomprised of two different layers: one that includes the stator and asecond that includes the rotor. In some embodiments, however, such aswhere the stator or the rotor is tilted permanently after manufacturing,a single layer may be used for both the stator and the rotor. Thestaggered mirrors may operate at their resonance frequency or at lowerfrequencies down to, and including, DC.

Traditional driving control circuitry for MEMS mirror devices requirescomplex processing and use of A/D converters, amplifiers, and filtersfor monitoring the mirror. Moreover, changing the laser light powercauses a change in mirror resonance frequency that traditional controlalgorithms are slow to detect and accommodate. Therefore, a need existsfor a simplified control system that addresses the deficiencies oftraditional MEMS mirror driving control circuitry.

SUMMARY

The present disclosure provides a control circuit for controllingoperation of a resonance MEMS mirror, the control circuit comprising:timing circuitry configured to control timing of activation pulses foroperating the MEMS mirror; amplifier circuitry configured to receive afirst control signal from the timing circuitry and, responsive thereto,to generate a first set of activation pulses for operating the MEMSmirror; and a detection circuit configured to detect current at the MEMSmirror and to generate a reset signal in response to detecting a changein the direction of the current at the MEMS mirror; wherein the timingcircuitry is further configured to terminate an activation pulse inresponse to receiving the reset signal from the detection circuit.

In another embodiment, the present disclosure provides a method forcontrolling operation of a resonance MEMS mirror, the method comprising:generating a first set of activation pulses for operating the MEMSmirror; generating a window for current detection, the windowoverlapping with an end of an activation pulse; detecting current at theMEMS mirror during the window; and terminating the activation pulse inresponse to detecting a change in current direction during the window.

The foregoing and other features and advantages of the presentdisclosure will become further apparent from the following detaileddescription of the embodiments, read in conjunction with theaccompanying drawings. The detailed description and drawings are merelyillustrative of the disclosure, rather than limiting the scope of theinvention as defined by the appended claims and equivalents thereof.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments are illustrated by way of example in the accompanyingfigures not necessarily drawn to scale, in which like numbers indicatesimilar parts, and in which:

FIG. 1 illustrates a schematic example of an embodiment of a MEMS mirrorapparatus;

FIG. 2 illustrates a graph of capacitance vs. mirror anglecharacteristics for an in-plane MEMS mirror embodiment;

FIG. 3 illustrates a graph of capacitance vs. mirror anglecharacteristics for a staggered MEMS mirror embodiment;

FIG. 4 illustrates a timing diagram showing example activation pulseinstances for the embodiment wherein the MEMS mirror is fabricated as anin-plane resonance mirror;

FIG. 5 illustrates an example timing diagram of activation pulseinstances for a staggered MEMS mirror embodiment where one pulse isapplied to each stator;

FIG. 6 illustrates an example timing diagram of activation pulseinstances for a staggered MEMS mirror embodiment where two pulses areapplied to each stator;

FIG. 7 illustrates a timing diagram for a staggered MEMS mirrorembodiment designed for a 6° opening angle with maximum capacitanceachieved at 3°;

FIG. 8 illustrates an example waveform for an example activation pulseand potential points of pulse termination that occur during a windowperiod opened towards the end of the pulse;

FIG. 9 illustrates a timing diagram of the waveform of FIG. 7 with thecounter values provided at T1on and T1off locations;

FIG. 10 illustrates a timing diagram for pulsing a staggered MEMS mirrorembodiment having two pulses applied to each stator;

FIG. 11 illustrates a timing diagram of the waveform of FIG. 7, showingthe second pulse for the second stator portion relative to the waveformand the respective counter values that correspond to the start and endof the pulse;

FIG. 12 illustrates a timing diagram showing the waveform of FIG. 4 andthe respective counter values that correspond to start (Ton) and end(Toff) of the pulse;

FIGS. 13A and 13B illustrate a schematic timing diagram for an exampleactivation pulse and a waveform representing the corresponding moment ofthe rotor caused by the pulse;

FIG. 14 illustrates a graph of normalized activation pulse lengthrelative to the ration between angle of maximum capacitance and maximumopening angle;

FIG. 15 illustrates an example timing diagram showing an enlargedactivation pulse and corresponding moment;

FIG. 16 illustrates a simulation for an embodiment of a controller forthe disclosed staggered MEMS mirror structure;

FIG. 17 illustrates a simulation of steady state operation of anembodiment of a controller for the disclosed staggered MEMS mirrorstructure having one pulse per stator;

FIG. 18 illustrates a simulation of steady state operation of anembodiment of a controller for the disclosed staggered MEMS mirrorstructure having two pulses per stator;

FIG. 19 illustrates an example circuit diagram for an embodiment of thedisclosed control system for an in-plane MEMS mirror; and

FIG. 20 illustrates an example circuit diagram for an embodiment of thedisclosed control system for a staggered MEMS mirror.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In the following detailed description and the attached drawings,numerous specific details are set forth to provide a thoroughunderstanding of the present disclosure. Those skilled in the art willappreciate, however, that the present disclosure may be practiced, insome instances, without such specific details. In other instances,well-known elements have been illustrated in schematic or block diagramform in order not to obscure the present disclosure in unnecessarydetail. Additionally, for the most part, specific details, and the like,have been omitted inasmuch as such details are not considered necessaryto obtain a complete understanding of the present disclosure, and areconsidered to be within the understanding of persons of ordinary skillin the relevant art.

It is further noted that, unless indicated otherwise, all functionsdescribed herein may be performed in hardware or as softwareinstructions for enabling a computer or other electronic device toperform predetermined operations, where the software instructions areembodied on a computer readable storage medium, such as RAM, a harddrive, flash memory or other type of computer readable storage mediumknown to a person of ordinary skill in the art. In certain embodiments,the predetermined operations of the computer, radio or other device areperformed by a processor such as a computer or an electronic dataprocessor in accordance with code such as computer program code,software, firmware, and, in some embodiments, integrated circuitry thatis coded to perform such functions. Furthermore, it should be understoodthat various operations described herein as being performed by a usermay be operations manually performed by the user, or may be automatedprocesses performed either with or without instruction provided by theuser.

The present disclosure provides a system and method for controlling aresonance MEMS mirror. When compared to conventional resonance MEMSmirror controllers, the disclosed control system provides a moreefficient, simplified design that reduces required circuitry andincreases operational speed.

Referring now to FIG. 1, a schematic example of a MEMS mirror 100 isshown. The MEMS mirror 100 comprises a tilting mirror (rotor) 115 havingrotor fingers 110 connected thereto, a MEMS structure (stator) 125having stator fingers 120 connected thereto, and a torsion spring 130that enables the rotor 115 to move. The MEMS mirror 100 may befabricated in two different varieties: (i) an in-plane design where therotor 115 and stator 125 reside in the same plane, and (ii) a staggereddesign where the rotor 115 and stator 125 reside on different planes.

The stator 125 includes a first group of stator fingers 120A located ona first portion 125A of the stator 125, and a second group of statorfingers 120B located on a second portion 125B of the stator 125, whereinthe first portion 125A of the stator 125 is opposite the second portion125B of the stator 125. It should be understood that the first andsecond stator portions 125A and 125B discussed herein may comprise asingle stator structure or, alternately, may comprise two separatestator structures. The groups of stator fingers are referenced by number120 when referred to collectively, and by the respective number 120A or120B when referenced separately.

Similarly, the rotor 115 includes a first group of rotor fingers 110Alocated on a first side of the rotor 115 and a second group of rotorfingers 110B located a second side of the rotor 115 opposite the firstside. These groups of rotor fingers are referenced by number 110 whenreferred to collectively, and by the respective number 110A or 110B whenreferenced separately.

In the view provided in FIG. 1, the rotor 115 is positioned such thatthe first group of rotor fingers 110A is interdigitated with the firstgroup of stator fingers 120A such that a capacitance exists between thefirst group of rotor fingers 110A and the first group of stator fingers120A. Similarly, the second group of rotor fingers 110B isinterdigitated with the second group of stator fingers 120B such that acapacitance exists between the second group of rotor fingers 110B andthe second group of stator fingers 120B. It should be understood that,in the embodiments provided herein, the rotor fingers 110 and statorfingers 120 are considered interdigitated regardless of the angle ofrotor 115.

In some embodiments, the first set of rotor fingers 110A may beinterdigitated with either the first or second sets of stator fingers120A/120B depending on the orientation of the rotor 115. Similarly, thesecond set of rotor fingers 110B may be interdigitated with either thefirst or second sets of stator fingers 120A/120B depending on theorientation of the rotor 115. In such embodiments, a capacitance may beachieved between the respective interdigitated sets of rotor fingers 110and stator fingers 120.

A MEMS structure, such as that shown in FIG. 1, can resonate in itsnatural frequency, which can be represented in accordance with thefollowing equation:

$f_{r} = {\frac{1}{2\;\pi}\sqrt{\frac{k}{j}}}$where f_(r) is the resonance frequency in Hz, k is the total torsionspring constant in N×m, and j is the rotor moment of inertia in kg×m².

The MEMS mirror 100 represents an embodiment that is activated byelectrostatic force. Accordingly, the mirror 100 implements a design,wherein the first group of stator fingers 120A are interdigitated withthe first group of rotor fingers 110A, and the second group of statorfingers 120B are interdigitated with the second group of rotor fingers110B. This design increases the capacitance between the rotor 115 andthe stator 125, which increases the electrostatic force generated whenapplying a voltage between the rotor 115 and stator 125.

As mentioned above, the MEMS mirror 100 may be fabricated in twodifferent varieties: (i) an in-plane design where the rotor 115 andstator 125 reside in the same plane, and (ii) a staggered design wherethe rotor 115 and stator 125 reside on different planes. FIG. 2illustrates a graph 200 of capacitance vs. mirror angle characteristicsfor the mirror 100 when fabricated as an in-plane MEMS mirror. In thisembodiment, line 202 represents the capacitance vs. mirror anglecharacteristics for the stator 125. As shown in FIG. 2, the greatestcapacitance reading is achieved when the rotor 115 is at angle zero,which places the rotor fingers 110 in-plane with the stator fingers 120.

FIG. 3 illustrates a graph 300 of the capacitance vs. mirror anglecharacteristics for the MEMS mirror 100 when fabricated as a staggeredMEMS mirror. That is, wherein the stator 125 and rotor 115 arefabricated on different planes. In this embodiment, line 302 representsthe capacitance vs. mirror angle characteristics for one portion of thestator (e.g., stator fingers 120A and first portion 125A), and line 304represents the capacitance vs. mirror angle characteristics for anotherportion of the stator (e.g., stator fingers 120B and second portion125B). As shown in FIG. 3, the greatest capacitance reading is not whenthe rotor 115 is at angle zero, but is instead at two different angles:a negative angle that achieves the greatest capacitance reading for thefirst set of stator fingers 120A and first portion 125A, and a positiveangle that achieves the greatest capacitance reading for the second setof stator fingers 120B and second portion 125B (or vice-versa).

Referring again to FIG. 1, the voltage applied between the stator 125and the rotor 115 creates an electrostatic force—more specifically,moment—that operates to rotate the rotor 115 in a direction thatincreases the capacitance between the stator 125 and the rotor 115. Insome embodiments, attraction forces are created when the rotor 115 andthe stator 125 are conducting currents. Because attraction forces aregenerated, the voltage applied between the stator 125 and the rotor 115may be pulsed when the rotor 115 is moving in a direction towardsgreater capacitance. Otherwise, if the voltage is pulsed at otherinstances, the forces will counteract the rotation of the rotor 115.

FIG. 4 provides a timing diagram 400 illustrating example activationpulse instances for the embodiment wherein the MEMS mirror 100 isfabricated as an in-plane resonance mirror. FIGS. 5 and 6 provide timingdiagrams illustrating example activation pulse instances for theembodiment wherein the MEMS mirror 100 is fabricated as a staggeredmirror.

In FIG. 4, timing of the pulses is shown relative to a waveform 402representing the mirror angle. In the embodiment illustrated in FIG. 4,both stator portions 125A and 125B are activated by the same pulse 404.A first pulse 404A is initiated at the peak of the waveform 402, whenthe mirror angle is at its greatest positive value. The first pulse 404Aterminates when the mirror angle is at 0°. The second pulse 404B isinitiated at the trough of the waveform 402, when the mirror angle is atits greatest negative value. The second pulse 404B terminates when themirror angle is at 0°. Finally, a third pulse 404C is shown similar tothe first pulse 404A. The third pulse 404C is initiated at the peak ofthe waveform 402, when the mirror angle is at its greatest positivevalue, and terminates when the mirror angle is at 0°.

In FIG. 5, the timing diagram 500 illustrates pulses shown relative to awaveform 502 representing the mirror angle, the angle of maximumcapacitance for the first stator portion 125A (represented by line 504),and the angle of maximum capacitance for the second stator portion 125B(represented by line 506). In this embodiment, a pulse 508 is applied tothe first stator portion 125A, commencing when the mirror angle is atits greatest positive value and terminating when the mirror anglereaches line 504, which represents the angle of maximum capacitance forthe first stator portion 125A. Similarly, pulse 510 is applied to thesecond stator portion 125B, commencing when the mirror angle is at itsgreatest negative value and terminating when the mirror angle reachesline 506, which represents the angle of maximum capacitance for thesecond stator portion 125A.

FIG. 6 illustrates a timing diagram 600 of another example embodimentfor pulsing the staggered mirror embodiment of the MEMS mirror 100,wherein the pulsing scheme provided in FIG. 5 is modified to includesecond pulses for each of the stator portions 125A/125B. The timing ofthe pulses is shown relative to the waveform 502 representing the mirrorangle, the angle of maximum capacitance for the first stator portion125A (represented by line 504), and the angle of maximum capacitance forthe second stator portion 125B (represented by line 506). In thisembodiment, two pulses (a primary pulse and a secondary pulse) areinitiated per cycle for each portion of the stator. Consistent with thetiming diagram 500 of FIG. 5, the first pulse 508 (a first primarypulse) is applied to the first stator portion 125A, commencing when themirror angle is at its greatest positive value and terminating when themirror angle reaches line 504, which represents the angle of maximumcapacitance for the first stator portion 125A. As shown in the timingdiagram 600 of FIG. 6, however, a second pulse 602 (a first secondarypulse) is applied to the first stator portion 125A, commencing when themirror angle is at its greatest negative value and terminating when themirror angle reaches line 504. As shown in FIG. 6, the second pulse 602(the secondary pulse) applied to the first stator portion 125A has ashorter duty cycle than the first pulse 508 (the primary pulse) appliedto the first stator portion 125A.

Similarly, and consistent with the timing diagram 500 of FIG. 5, thefirst pulse 510 (a second primary pulse) is applied to the second statorportion 125B, commencing when the mirror angle is at its greatestnegative value and terminating when the mirror angle reaches line 506,which represents the angle of maximum capacitance for the second statorportion 125B. As shown in the timing diagram 600 of FIG. 6, however, asecond pulse 604 (a second secondary pulse) is applied to the secondstator portion 125B, commencing when the mirror angle is at its greatestpositive value and terminating when the mirror angle reaches line 506.As shown in FIG. 6, the second pulse 604 (the secondary pulse) appliedto the second stator portion 125B has a shorter duty cycle than thefirst pulse 510 (the primary pulse) applied to the second stator portion125B.

As shown in FIG. 6, the second pulses 602 and 604 are applied to therespective first and second stator portions 125A and 125B when theabsolute value of the mirror opening angle is greater than the angle ofmaximum capacitance for the respective stator portion 125A/125B.

In the embodiments discussed herein, MEMS mirrors have a quality factor(Q) that is relatively high and controlled primarily by air friction. Atatmospheric pressure, a typical quality factor could be in the range ofseveral hundreds to one thousand, whereas packaging the mirror in a lowpressure environment may increase the quality factor to tens ofthousands or even over hundreds of thousands. While at low qualityfactor (e.g., in atmospheric pressure), a control system may berelatively simple because the mirror may have bandwidth of only a fewHertz. Conversely, at high quality factor, the control system becomesmore complex with a bandwidth that is fractions of a Hertz that isneeded to track temperature changes that may cause variations of tens ofHertz.

One method for monitoring the crossover point (i.e., the point in timeat which the mirror is parallel to the plane, that is, having a mirrorangle of 0°) of an in-plane mirror is described in U.S. Pat. No.8,553,308, which is incorporated herein by reference. In U.S. Pat. No.8,553,308, the crossover point is detected by monitoring the currentthrough the stators. The current i through a stator is represented bythe following equation:

$\begin{matrix}{i = {\frac{dq}{dt} = {\frac{d\left( {\left( {c_{p} + c} \right)v} \right)}{dt} = {{c\frac{d(v)}{dt}} + {v\frac{d\left( {c_{p} + c} \right)}{dt}}}}}} & {{Equation}\mspace{14mu}(1)}\end{matrix}$where c_(p) is the parasitic capacitance of the wiring, which isconstant, v represents the voltage, and c is the capacitance.Accordingly, Equation (1) can be reduced to the following:

$i = {{c\frac{d(v)}{dt}} + {v{\frac{d(c)}{dt}.}}}$

To eliminate the switching effect of v, U.S. Pat. No. 8,553,308 proposesusing only one pulse per mirror cycle and adding a small DC voltageduring the remainder of the time for monitoring

$v{\frac{d(c)}{dt}.}$However, this proposed method of monitoring the crossover point is notsuitable for both in-pane and staggered MEMS mirror embodiments.

The system and method for a MEMS mirror control system proposed hereinis suitable for both in-plane mirror embodiments as well as staggeredmirror embodiments. It is also suitable for high quality factor mirrorsthat usually involve complex electronics for tracking the mirroroperating frequency.

In accordance with the present disclosure, the current through therelevant stator portion is monitored to determine when to terminate thepulse applied to the respective stator portion. Referring again toEquation (1), and assuming a steady state with a driving pulse v, with vbeing constant at the moment, the current i through the stator is:

$i = {v{\frac{d(c)}{dt}.}}$When approaching the maximum capacitance,

$\frac{d(c)}{dt}$is positive and/is positive. At the moment after reaching maximumcapacitance, i becomes negative. Put differently, considering theperspective of the power supply, as long as the power supply current ispositive, sourcing current from the positive terminal, the power supplytransfers energy to the load. At the moment the current is movingtowards the positive terminal, the power supply receives the energy. Thepulse should be switched off at the moment the current i becomesnegative.

As discussed below, the disclosed control system creates a windowpositioned near the end of each pulse for monitoring and detecting adirection change of the current i. Once this direction change isobserved, the pulse is terminated at that time. If no change ofdirection is detected, the pulse terminates at the end of its standardpulse cycle as discussed above with respect to FIGS. 4-6. In otherwords, if the direction change occurs during the window, the pulse isterminated early—at the time of the direction change—otherwise the pulseterminates as indicated above.

In some embodiments, the disclosed mirror control system is based on afree-running clock that provides the timing pulses for the mirror 100. Aten-stage counter (1024 clock cycles counting from 0 to 1023) may beimplemented in some embodiments to provide sufficient resolution fortiming manipulations. For example, FIG. 7 provides a timing diagram 700for a staggered mirror embodiment of the MEMS mirror 100 designed for+/−6° opening with maximum capacitance achieved at +/−3°. The timingdiagram 700 is similar to that shown in FIG. 5, and illustrates thepulses applied to the stator portions 125A/125B for the +/−3° and +/−6°angle values.

As shown in FIG. 7, timing of the pulses is shown relative to a waveform702 representing the mirror angle, the −3° angle of maximum capacitancefor the first stator portion 125A (represented by line 704), and the 3°angle of maximum capacitance for the second stator portion 125B(represented by line 706). In this embodiment, the MEMS mirror 100 isdesigned for +/−6° opening. Accordingly, the waveform 702 has a peakvalue of 6° and a trough value of −6°. A pulse 708 is applied to thefirst stator portion 125A, commencing at T1on when the mirror angle isat 6° and terminating at T1off when the mirror angle reaches −3° at line704, which represents the angle of maximum capacitance for the firststator portion 125A. Similarly, pulse 710 is applied to the secondstator portion 125B, commencing at T2on when the mirror angle is at −6°and terminating at T2off when the mirror angle reaches 3° at line 706,which represents the angle of maximum capacitance for the second statorportion 125A.

The duty cycle of each pulse is controlled in accordance with thecounter. The counter is designed to complete counting at a slowerfrequency than the lowest resonance frequency that is accepted for themirror 100, when accounting for the design of the mirror and anyacceptable variance. For example, if the mirror 100 is designed for 20KHz operation with a variance of +/−700 Hz, the counter should bedesigned slightly lower than 19.3 KHz to account for temperature andother environmental variations. For example, setting the frequency to19.2 KHz should be sufficient to account for small frequency variationsthat result from various factors such as temperature changes and airviscosity.

In accordance with the present disclosure, a window is opened near theend of each pulse to monitor the current through the relevant statorportion 125A/125B. At the moment the current becomes negative, thecounter is set to a specific value to indicate termination of the pulse,as discussed in greater detail below.

In some embodiments, a count from 0 to 1023 provides for one mirrorcycle, accounting for both pulse 708 and pulse 710, wherein each pulseis 512 clock cycles. It should be appreciated, however, the exact designmay vary for different implementations. In the example illustrated inFIG. 7, the clock starts at count zero at T1on and ends at count 1023 atT2off. The counter resets to zero immediately thereafter. Because thecounter is slower than the actual mirror resonance rate, “T1off” occursat timer value of 511 or sooner and “T2off” occurs at timer value 1023or sooner.

Towards the end of each pulse (i.e., pulse 708 and pulse 710), a windowis opened to monitor the current across the relevant stator portion125A/125B. When the current changes direction (becomes negative), thecounter is set to the relevant defined value to indicate the end of therespective pulse 708/710. For example, if the window is opened towardsthe end of pulse 708 (i.e., at T1off), the current (i1) of the firststator portion 125A is monitored. When current i1 becomes negative, thecounter is set to the value of 512. Similarly, if the window is openedat the end of pulse 710 (i.e., at T2off), the current (i2) of the secondstator portion 125B is monitored. When current i2 becomes negative, thecounter is set to a value of 0.

FIG. 8 illustrates an example waveform 800 for an example activationpulse 802 and potential points 804 of pulse termination that occurduring a window period 806 opened towards the end of the pulse 802. Thepoints 804 of termination represent potential instances of the counterwhere the current change could occur and the pulse 802 would thenterminate. If no current change is detected, the pulse 802 terminates atthe end 808 of the window 806. It should also be appreciated that alimited window reduces susceptibility to erroneous detection in noisyenvironments.

In some embodiments, the counter values of the T1on and T1off times maybe calculated in accordance with the following equations. The pulselength is equal to the time from the positive peak in FIG. 7 to thepoint of negative maximum capacitance. In accordance with the embodimentillustrated in FIG. 7, the maximum opening angle is +/−6°, and theposition of maximum capacitance is +/−3°. Mirror function is representedby the following equation:θ(t)=θ_(max) sin(2πft)Where θ_(max)=6°, and f=20 KHz. This formula may be used to determinethe tame lapsed (t) from θ(t)=6° to θ(t)=−3°. Accordingly,

${t = {{\frac{1}{2\;\pi\; f}{{asin}\left( \frac{\theta(t)}{\theta_{\max}} \right)}} = {\frac{T}{2\;\pi}{{asin}\left( \frac{\theta(t)}{\theta_{\max}} \right)}}}},$where T is the cycle time of the mirror. Substituting the respectiveangles yields the following equation:

$\begin{matrix}{{\Delta\; t} = {{\frac{T}{2\;\pi}\left\lbrack {{{asin}\left( \frac{- 3}{6} \right)} - {{asin}\left( \frac{6}{6} \right)}} \right\rbrack} = {\frac{T}{2\;\pi}\left\lbrack {{{asin}\left( {- 0.5} \right)} - {{asin}(1)}} \right\rbrack}}} & {{Equation}\mspace{14mu}(2)}\end{matrix}$which may be simplified as:

${\Delta\; t} = {{\frac{T}{2\;\pi}\left\lbrack {\pi + \frac{\pi}{6} - \frac{\pi}{2}} \right\rbrack}.}$In Equation (2)

$\frac{\pi}{2}$represents T1on at the maximum opening angle, and

$\pi + \frac{\pi}{6}$represents T1off at the position of maximum capacitance, as shown inFIG. 9. FIG. 9 illustrates a timing diagram 900 of the waveform 702 ofFIG. 7 with the counter values provided at the T1on and T1off locations.

Finally, this equation can be further simplified as follows:

${\Delta\; t} = {{\frac{T}{2\;\pi}\left\lbrack \frac{2\;\pi}{3} \right\rbrack} = {\frac{T}{3}.}}$

For a counter of 1024 cycles, the count for a pulse commencing at T1onand terminating at T1off is:

${{\Delta\; N} = {\frac{1024}{3} \approx 341}},$where ΔN represents the logical value of the counter in bits, whichrepresents the corresponding time in seconds.

With reference to the timing diagrams 700 and 900 in FIGS. 7 and 9,respectively, the counter is set to 512 (half of the counter's maximumcount) to indicate the normal termination point of pulse 708. Thus,T1off

512, and T1on=512−341=171. Pulse 708 commences at clock cycle 171, andterminates at about 512, having a length of 341 clock cycles. T2on andT2off may be calculated in a similar manner. Assuming the T2off countervalue is 1024=0, and T2on occurs exactly one half mirror cycle laterthan T1off, T2on is as follows: T2on=1024−341=683.

As previously mentioned, the free-running clock should cover the lowestresonance frequency accommodated by the mirror 100. Again, assuming adesigned resonance value of 20 KHz with a manufacturing tolerance of+/−500 Hz, and assuming temperature and other environmental conditionsmay vary tolerance by +/−50 Hz, the resonance frequency could be a valuebetween 19,450 Hz and 20,550 Hz. Accordingly, the clock used for thecounter should have an operating frequency of 19450×1024≈19.916 MHz.

Referring briefly to FIGS. 7 and 8, the length of the window 806 shouldbe such that it accommodates the fastest resonance frequency of themirror 100. In the above example, this is 20,550 Hz. Accordingly, thepulse length for the fastest resonance is 341×(19450/20550)≈322 clockcycles. Thus, for a pulse length of 341 clock cycles, the window shouldbegin at clock cycle 322 of the pulse (i.e., the 322^(nd) clock cyclesince the pulse commenced) and close at the earlier of: (i) detection ofa negative current on the respective stator, or (ii) clock cycle 431 ofthe pulse (i.e., the 431^(st) clock cycle since the pulse commenced).Applying this range to pulse 708 of FIG. 7, the window opens at clockcycle 493 and closes at clock cycle 512, which is the end of the pulse708, assuming the current i1 does not become negative during the window(i.e., clock cycles 493-512). It should be understood from the foregoingthat similar calculations may be performed to calculate the window forthe pulse 710 applied to the second stator portion 125B.

Just as the timing diagram 500 of FIG. 5 was modified in FIG. 6 toaccommodate a second pulse for each stator portion, so too can thetiming diagram 700 of FIG. 7. This modification is illustrated by thetiming diagram 1000 shown in FIG. 10.

FIG. 10 illustrates a timing diagram 1000 similar to that provided inFIG. 6 for pulsing the staggered mirror embodiment of the MEMS mirror100, wherein the pulsing scheme provided in FIG. 7 is modified toinclude second pulses (secondary pulses) for each of the stator portions125A/125B. The timing of the primary and secondary pulses is shownrelative to the waveform 702 representing the mirror angle, the angle ofmaximum capacitance for the first stator portion 125A (represented byline 704), and the angle of maximum capacitance for the second statorportion 125B (represented by line 706). In this embodiment, two pulsesare initiated per cycle for each portion of the stator. Consistent withthe timing diagram 700 of FIG. 7, the first pulse 708 (a first primarypulse) is applied to the first stator portion 125A, commencing when themirror angle is at 6° and terminating when the mirror angle reaches −3°at line 504, which represents the angle of maximum capacitance for thefirst stator portion 125A. As shown in the timing diagram 1000 of FIG.10, however, a second pulse 1002 (a first secondary pulse) is applied tothe first stator portion 125A, commencing when the mirror angle is atits lowest value)(−6° and terminating when the mirror angle reaches −3°at line 504. As shown in FIG. 10, the second pulse 1002 (the secondarypulse) applied to the first stator portion 125A has a shorter duty cyclethan the first pulse 708 (the primary pulse) applied to the first statorportion 125A.

Similarly, and consistent with the timing diagram 700 of FIG. 7, thefirst pulse 710 (a second primary pulse) is applied to the second statorportion 125B, commencing when the mirror angle is at its lowest value(−6°) and terminating when the mirror angle reaches 3° at line 706,which represents the angle of maximum capacitance for the second statorportion 125B. As shown in the timing diagram 1000 of FIG. 10, however, asecond pulse 1004 (a second secondary pulse) is applied to the secondstator portion 125B, commencing when the mirror angle is at its greatestvalue (6°) and terminating when the mirror angle reaches 3° at line 706.As shown in FIG. 10, the second pulse 1004 (the secondary pulse) appliedto the second stator portion 125B has a shorter duty cycle than thefirst pulse 710 (the primary pulse) applied to the second stator portion125B.

The foregoing disclosure can also be implemented to generate windows forthe second pulses provided in the embodiment described above andillustrated in FIG. 10. For example, FIG. 11 illustrates a timingdiagram 1100 of the waveform 702 similar to that shown in FIG. 9. Inaddition to what is provided in FIG. 9, the timing diagram 1100 showsthe second pulse 1004 for the second stator portion 125B relative to thewaveform 702 and the respective counter values that correspond to thestart and end of the pulse 1004.

As shown in FIG. 11, the on time of the pulse 1004 is the same as T1on,which is counter value 171. The pulse 1004 terminates when the mirrorangle is 3°. Therefore, the length of pulse 1004 is the time it takesfor the mirror to change its angle from 6° to 3°. The mirror equation isθ(t)=θ_(max) sin(2πft). So, with θ_(max)=6°, the time it takes for asine wave to change from a value of 1 to 0.5 is

$\frac{\pi}{3}.$Thus, the length of pulse 1004 is

$\frac{\pi}{3},$which is equal to a count of approximately 170 clock cycles.Accordingly, counting will commence for the pulse 1004 at a countervalue of 171, and will terminate at a value of 341.

Although it is not shown in FIG. 11, it should be appreciated that thesecond pulse of the first stator portion 125A (see pulse 1002 in FIG.10), is delayed relative to the pulse 1004 in FIG. 10 by half a mirrorcycle, or 512 clock cycles. Thus, the second pulse of the first statorportion 125A (pulse 1002) will commence at counter value 683(171+512=683) and will terminate at counter value 853 (683+170=853).

Again assuming a clock operating at 19.916 MHz, the length of the windowfor the second pulses (i.e., pulses 1002 and 1004) should be such thatit accommodates the fastest resonance frequency of the mirror 100. Inthe above example, this is 20550 Hz. Accordingly, for the lengths of thesecond pulses (170 clock cycles each), the fastest resonance is170×(19450/20550) 161 clock cycles. Thus, for a pulse length of 170clock cycles, the window should begin at clock cycle 161 of the pulse(i.e., the 161^(st) clock cycle since the pulse commenced) and close atthe earlier of: (i) detection of a negative current on the respectivestator, or (ii) clock cycle 170 of the pulse (i.e., the 170^(th) clockcycle since the pulse commenced). Applying this range to pulse 1004 ofFIGS. 10 and 11, the window opens at clock cycle 332 and closes at clockcycle 341, which is the end of the pulse 1004, assuming the current i2does not become negative during the window (i.e., clock cycles 332-341).It should be understood from the foregoing that similar calculations maybe performed to calculate the window for the second pulse 1002 appliedto the first stator portion 125A.

Because activating the second pulses 1002/1004 from the start may createambiguity (both stator portions are operating), it may be desirable insome embodiments to enable the second pulses 1002/1004 after the mirroralmost achieves its working opening angle (e.g., when the mirror is at80%-90% of the required opening angle). Then, when steady state isachieved with one pulse, the second pulse could be activated to accountfor the remaining 10%-20% of the opening angle.

It should be appreciated that the above disclosure may also be appliedto the in-plane embodiment of the MEMS mirror 100. Accordingly, thetiming of the pulses 404 provided in FIG. 4 may be calculated similar tothe pulses 708 and 710, except that the pulses 404 terminate at the zerocrossing. FIG. 12 provides an example timing diagram 1200 showing thewaveform 402 of FIG. 4 and the respective counter values that correspondto start (Ton) and end (Toff) of the pulse 404.

Referring now to FIGS. 4 and 12, pulse 404 commences at Ton, which is atthe maximum opening angle of the waveform 402. In accordance with theabove example where this angle is 6°, the counter value is 171 at Ton.The pulse 404 terminates at Toff when the mirror angle is 0°. The pulselength is

$\frac{\pi}{2},$which is equal to a count of approximately 255 clock cycles. Thus, thepulse 404 has a length of approximately 255 clock cycles. Therefore,counting will commence for the pulse 404 at a counter value of 171, andwill terminate at a value of about 426.

Again assuming a clock operating at 19.916 MHz, the length of the windowfor each of the pulses 404 should be such that it accommodates thefastest resonance frequency of the mirror 100. In the above example,this is 20550 Hz. Accordingly, for the length of the pulse 404 (255clock cycles), the fastest resonance is 255×(19450/20550) 241 clockcycles. Thus, for a pulse length of 255 clock cycles, the window shouldbegin at clock cycle 241 of the pulse (i.e., the 241^(st) clock cyclesince the pulse commenced) and close at the earlier of: (i) detection ofa negative current on the stator, or (ii) clock cycle 255 of the pulse(i.e., the 255^(th) clock cycle since the pulse commenced). Applyingthis range to pulse 404 of FIGS. 4 and 12, the window opens at clockcycle 412 and closes at clock cycle 426, which is the end of the pulse404, assuming the current i does not become negative during the window(i.e., clock cycles 412-426).

In some embodiments, the disclosed control system is capable ofcontrolling the opening angle of a staggered embodiment of the MEMSmirror 100.

Referring now to FIG. 13A, a schematic timing diagram 1300 is shown foran example activation pulse 1302 and a waveform 1304 representing thecorresponding moment of the rotor 115 caused by the pulse 1302. At thestart of the pulse 1302, the moment 1304 is low due to the distancebetween the stator portion 125A/125B and the rotor fingers 110, whichare located apart from each other. As the rotor fingers 110 approach therespective stator portion 125A/125B, the moment 1304 increases and, atthe point of maximum capacitance, it quickly switches from positivecurrent 1304A to negative current 1304B.

As discussed herein, the pulse 1302 is switched off as soon as thenegative current 1304B is sensed. Because this current detection andsubsequent switching cannot be done in zero time, there exists aninherent delay before the pulse 1302 is switched off. This delay 1305,shown in FIG. 13B, results in a small deviation from the optimal caseshown in FIG. 13A. This small delay is usually negligible.

According to Equation (1), the measured current has two components: onerelating to a change in voltage and another relating to a change incapacitance. When a window is generated, the change in voltage isnegligible and all transients subside, so the current measurement reliesprimarily on the change in capacitance.

As demonstrated in Equation (2), the optimal length of the pulse mayvary according to the opening angle of the mirror. For an opening angleof 6°, Equation (2) provides the following calculations:

${\Delta\; t} = {\frac{T}{2\;\pi}\left\lbrack {{{asin}\left( \frac{- 3}{6} \right)} - {{asin}\left( \frac{6}{6} \right)}} \right\rbrack}$

In general, Δt is a function of the opening angle θ_(max). Thus:

${\Delta\;{t\left( \theta_{\max} \right)}} = {\frac{T}{2\;\pi}\left\lbrack {{{asin}\left( \frac{- 3}{\theta_{\max}} \right)} - {{asin}(1)}} \right\rbrack}$

Accordingly, the greater the opening angle θ_(max), the shorter Δt. Thisrelationship is illustrated in FIG. 14, which shows a graph 1400 ofnormalized activation pulse length relative to the ratio between angleof maximum capacitance and maximum opening angle.

In the above example, the ratio was 3/6=0.5, and the result was0.3333N=0.3333×1024≈341.

At the start of the activation pulse 1302 shown in FIG. 13B, it isdifficult to measure the current component of Equation (1) because themoment 1304 at that point is very low and the current, which isproportional to the moment, is also very low. Additionally, theactivation pulse causes transients due to the need to charge all straycapacitances. This transient is on the order of 1 μs, because therelated voltage component will mask the signal due to a voltage stepfunction. Assuming the transient, which occurs during the rise time ofthe pulse 1302, takes time of Ttransient, it is possible to increase theactivation pulse 1302 to a slightly longer value than Ttransient and tothen measure the current at the front end of the pulse 1302 and again atthe end of the pulse 1302. Because the moment 1304 is low at thebeginning of the pulse 1302, the efficiency reduction will be low. FIG.15 provides an example timing diagram 1500 showing an activation pulse1502 and moment 1504 similar to those provided in FIG. 13B. In FIG. 15,however, the activation pulse 1502 is enlarged compared to that providedin FIG. 13B.

In the example timing diagram 1500 in FIG. 15, the transient time takes21 clock cycles. Additionally, there is an uncertain amount of time forwhich the pulse 1502 could terminate early, due to the window openednear the end of the pulse 1502. In accordance with the examples providedherein, this window occurs from clock cycles 493 through 512 (19 clockcycles). The 19 clock cycles for the window are added to the 21 clockcycles located at the front end of the pulse 1502 to produce 40 clockcycles. The 40 clock cycles are added to the original pulse length of341 clock cycles. Thus, the pulse 1502 is enlarged to a total length of381 clock cycles as shown in FIG. 15.

Opening angle control may be implemented to ensure the mirror openingangle is correct and stable during environment changes such as airpressure variation. As explained herein, the opening angle control iscoordinated with the current, specifically, the timing of positivecurrent detection. The duration of the positive current at the statormay be measured to determine whether to increase or decrease theactivation pulse 1502. The 40 clock cycles located at the front end ofthe pulse 1502 provide a transition to the point of opening anglecontrol located at 1506.

In accordance with the example provided herein, the positive currentshould be exactly 341 clock cycles to ensure that the required openingangle is correct. In an effort to effect this timing, the counter timeis measured at the termination of the pulse 1502 to determine if thepulse was 341 clock cycles. If the value of the counter at the off timeindicates the pulse exceeds 341 clock cycles, the opening angle is lowerthan what is required. Conversely, if the value indicates the pulse wasshorter than 341 clock cycles, the angle is opened more than required.The desired opening angle can be achieved by changing the value of theactivating voltage according to whether the timing of the pulse isgreater than 341 or less than 341 clock cycles. If the timing of thepulse is less than the desired length of 341 clock cycles, theactivating voltage of the subsequent pulse is decreased. If the timingof the pulse is greater than the desired length of 341 clock cycles, theactivating voltage of the subsequent pulse is increased.

Because the off-time value of the counter cannot be predicted before apulse commences, the counter value associated with the off-time of theprevious pulse is registered for the next cycle. In other words, thetiming of the previous pulse is used to determine the voltage amplitudefor the next pulse. For example, suppose the actual reset time of theprevious pulse cycle happens exactly at a counter value of 500 (i.e.,the current is detected to be negative at counter value 500). The nextpulse cycle will produce a window at the beginning of the nextactivation pulse to monitor the current at the counter value of500−341=159 (i.e., the 159^(th) counter value of the next activationpulse). If the current is negative at counter value 159, the openingangle is too high, and the activation voltage for the subsequentactivation pulse should be reduced. If the value is positive, theopening angle is too low, and the activation voltage for the subsequentactivation pulse should be increased.

FIG. 16 provides a graph 1600 of various waveforms corresponding to asimulation performed in accordance with the embodiments describedherein. Specifically, the resonance frequency was set to 20 KHz, theangle of maximum capacitance was set to 3°, and the opening anglecontrol system was set to 6V, with a Q of approximately 8150. FIG. 16illustrates operation of the opening angle controller. The values 1602and line 1603 represent the opening angle in degrees, and the values1604 and line 1605 represent the activation voltage applied. Theactivation voltage was set to alternate between 80V and 120V. In thefirst 76 ms of the rise-up, the activation voltage was consistently at120V. When the mirror achieves its designed opening value, the controlsystem maintains constant opening angle by alternating between the twoactivation voltages.

FIG. 17 provides a graph 1700 of various waveforms corresponding to asimulation performed in accordance with the embodiments describedherein. The graph 1700 illustrates three cycles during a steady state.The values 1702 and line 1703 represent the mirror angle, values 1704and line 1705 represent the Stator 1 activation pulse, and the values1706 and line 1707 represent the counter value. As shown in FIG. 17, thepulse value alternates between 80V and 120V. The counter sets to 512 atpoints 1710, and to a value of 0 at points 1715.

FIG. 18 provides a graph 1800 of various waveforms corresponding to asimulation performed in accordance with the embodiments describedherein. The graph 1800 is similar to that shown in FIG. 17, but providesa steady state having two pulses per stator. The second pulses 1802 areshorter than the first pulses 1804. Additionally, the alternatingactivation voltages are set to 60V and 100V to provide improvedefficiency.

FIG. 19 illustrates an example controller circuit 1900 for controllingoperation of an in-plane MEMS mirror in accordance with the presentdisclosure. The controller 1900 is shown connected to an in-plane MEMSmirror 1901, similar to the MEMS mirror 100 shown in FIG. 1.Specifically, a high-voltage amplifier 1902 is coupled to the stator ofthe in-plane mirror 1901 for providing an activation pulse in accordancewith the present disclosure. A comparator 1903 is coupled to the rotorof the in-plane mirror 1901 and is used to detect the current directionas discussed herein. Alternatively, the high-voltage amplifier 1902could be connected to the rotor and the comparator 1903 could beconnected to the stator portion of the MEMS mirror 1901. The circuit1900 also includes logic circuitry 1904 (e.g., an AND gate) connected tothe output of the comparator 1903, and a timing circuit 1905 forcontrolling the timing of the activation pulses and the windows, asdiscussed herein. The comparator 1903 and logic circuitry 1904 comprisea detection circuit 1915 for monitoring the current at the mirror 1901,detecting a change in the current direction, and generating a resetsignal 1908 for the timing circuit 1905.

In some embodiments, the timing circuit 1905 is a free-runningresettable multivibrator or counter. The timing circuit 1905 produces anactivation pulse control signal 1906 and a window activation signal1907, and receives a reset signal 1908 from the logic circuitry 1904.When the reset signal 1908 is low, meaning there is no “timer reset,”the timing circuit 1905 provides steady 50% duty cycle activation signalpulses 1906 at the lowest possible resonance frequency of the in-planemirror 1901 according to manufacturing tolerances. When the reset signal1908 is high (this happens during pulse activation, as explainedherein), the timing circuit 1905 disables the activation pulse 1906 andrestarts a new cycle, beginning with no activation signal.

The timing circuit generates the window activation signal 1907 toactivate the logic circuitry 1904 during the possible appearance of anegative current. The window activation signal 1907 is generated at afrequency corresponding to the fastest possible resonance frequency ofthe in-plane mirror 1901 according to the manufacturing tolerances. Thelogic circuitry 1904 will be deactivated (i.e., the window signal 1907will terminate) simultaneously with the end of the pulse activationsignal 1906 (this could occur due to timer reset or the free-runningoperation of the timing circuit 1905). Accordingly, the window signal1907 is only activated when the activation signal 1906 is activated. Theoperation of the window signal 1907 is intended to reduce erroneousnegative current detection signals. Such erroneous signals areespecially likely to appear during start of operation when currents aresmall and noisy.

The high voltage amplifier 1902 converts the activation signal 1906 tothe high voltage level needed by the MEMS mirror 1901 to apply theactivation pulses to the stator of the mirror 1901.

In some embodiments, the comparator 1903 is a transimpedance amplifier(TIA) with low impedance to ground. The comparator 1903 detects thedirection of the current 1909 that flows to ground. When the currentdirection is towards the comparator 1903, the output 1910 will be low.When the current 1909 reverses and flows from the comparator 1903 to thein-plane MEMS mirror 1901, the output 1910 of the comparator 1903becomes high to signal detection of a negative current. The output 1910is further gated by the logic circuitry 1904 (e.g., AND gate) to ensurethe reset signal 1908 is generated during the appropriate timing (i.e.,during a window). The logic circuitry 1904 then produces the resetsignal 1908 to reset the timer 1905.

FIG. 20 illustrates an example controller circuit 2000 for controllingoperation of a staggered MEMS mirror 2001 in accordance with the presentdisclosure. The staggered MEMS mirror 2001 requires two differentactivation pulses—one for each stator portion. The controller providedin FIG. 20 is similar to the controller 1900 in FIG. 19, except that thecontroller 2000 is connected to a staggered MEMS mirror 2001, andincludes a second high voltage amplifier 2002. The first high voltageamplifier 1902 is connected to a first stator portion of the mirror2001, and the second high voltage amplifier 2002 is connected to asecond stator portion of the mirror 2001. The first high voltageamplifier 1902 receives the activation signal 1906 to generate the firstactivation pulses for the mirror 2001. Specifically, the first amplifier1902 converts the first activation signal 1906 to the high voltage levelneeded by the MEMS mirror 2001 to apply the activation pulses to thefirst stator portion of the mirror 2001. The second high voltageamplifier 2002 receives a second activation signal 2003 from the timingcircuit 1905 and generates the second activation pulses for the mirror2001. Specifically, the second amplifier 2002 converts the secondactivation signal 2003 to the high voltage level needed by the MEMSmirror 2001 to apply the second activation pulses to the second statorportion of the mirror 2001. Generally, due to manufacturing processrestrictions, there is one joint rotor for both stators. Thus, the rotorcould be connected to a single comparator 1903 which is used forcontrolling both stators. The comparator 1903 detects the current of theone, activated stator at the detection time.

As disclosed herein, the controller circuit 2000 may be used to provideonly one pulse per stator portion per cycle as discussed with referenceto FIGS. 5 and 7. In other embodiments, the controller circuit 2000 maybe used to provide two pulses per stator portion per cycle as discussedwith reference to FIGS. 6 and 10. In this embodiment, the first pulsesare provided by the first activation signal 1906 and first high voltageamplifier 1902, and the second pulses are provided by the secondactivation signal 2003 and second high voltage amplifier 2002.Accordingly, the window signal 1907 is only active when either the firstor second activation signals 1906 and 2003 are active.

In the embodiment illustrated in FIGS. 19 and 20, it should beappreciated that the connections to the stator and rotor may be swappedsuch that the amplifier circuitry 1902 and 2002 are coupled to the rotorand the comparator 1903 is connected to the stator portion. This isbecause the circuit between the stator and rotor is essentially avariable capacitor.

The foregoing description has provided by way of exemplary andnon-limiting examples a full and informative description of one or moreexemplary embodiments of this invention. However, various modificationsand adaptations may become apparent to those skilled in the relevantarts in view of the foregoing description when read in conjunction withthe accompanying drawings and the appended claims. However, all such andsimilar modifications of the teachings of this invention will still fallwithin the scope of this invention as defined in the appended claims.

What is claimed is:
 1. A control circuit for controlling operation of aresonance MEMS mirror, the control circuit comprising: timing circuitryconfigured to control timing of activation pulses for operating the MEMSmirror; amplifier circuitry configured to receive a first control signalfrom the timing circuitry and, responsive thereto, to generate a firstset of activation pulses for operating the MEMS mirror; and a detectioncircuit configured to detect current at the MEMS mirror and to generatea reset signal in response to detecting a change in the direction of thecurrent at the MEMS mirror; wherein the timing circuitry is furtherconfigured to terminate an activation pulse in response to receiving thereset signal from the detection circuit.
 2. The control circuit of claim1, wherein the MEMS mirror is an in-plane MEMS mirror.
 3. The controlcircuit of claim 1, wherein the MEMS mirror is a staggered MEMS mirror.4. The control circuit of claim 1, wherein the detection circuitcomprises: comparator circuitry configured to detect the direction ofthe current at the MEMS mirror and to generate a signal to indicate thechange in the direction of the current at the MEMS mirror; and logiccircuitry configured to receive the signal from the comparator circuitryand, responsive thereto, to generate the reset signal.
 5. The controlcircuit of claim 1, wherein the first set of activation pulses includesa first primary pulse applied to a first portion of the MEMS mirror anda second primary pulse applied to a second portion of the MEMS mirror.6. The control circuit of claim 5, wherein the first primary pulsecommences when an angle of the MEMS mirror is at its greatest positivevalue and terminates at an angle of maximum capacitance for the firstportion of the MEMS mirror, and wherein the second primary pulsecommences when the angle of the MEMS mirror is at its greatest negativevalue and terminates at an angle of maximum capacitance for the secondportion of the MEMS mirror.
 7. The control circuit of claim 1, whereinthe amplifier circuitry is further configured to receive a secondcontrol signal from the timing circuitry and, responsive thereto, togenerate a second set of activation pulses for operating the MEMSmirror.
 8. The control circuit of claim 7, wherein the second set ofactivation pulses includes a first secondary pulse applied to a firstportion of the MEMS mirror and a second secondary pulse applied to asecond portion of the MEMS mirror.
 9. The control circuit of claim 8,wherein the first secondary pulse commences when an angle of the MEMSmirror is at its greatest negative value and terminates at an angle ofmaximum capacitance for the first portion of the MEMS mirror, andwherein the second secondary pulse commences when the angle of the MEMSmirror is at its greatest positive value and terminates at an angle ofmaximum capacitance for the second portion of the MEMS mirror.
 10. Thecontrol circuit of claim 7, wherein each of the activation pulses in thesecond set have a shorter duty cycle than each of the activation pulsesof the first set.
 11. The control circuit of claim 1, wherein the timingcircuitry is further configured to control timing of a window signal foractivating the detection circuit to monitor the current at the MEMSmirror.
 12. The control circuit of claim 11, wherein the timingcircuitry is configured to activate the detection circuit near the endof an activation pulse to monitor the current at the MEMS mirror. 13.The control circuit of claim 1, wherein the timing circuitry isconfigured to measure a length of a first activation pulse and, based onthe measurement, to adjust a voltage of a subsequent activation pulse.14. The control circuit of claim 13, wherein the voltage of thesubsequent activation pulse is decreased if the length of the firstactivation pulse is less than a desired length, and wherein the voltageof the subsequent activation pulse is increased if the length of thefirst activation pulse is greater than the desired length.
 15. Thecontrol circuit of claim 14, wherein the length of the first activationpulse and the desired length are measured in clock cycles.
 16. Thecontrol circuit of claim 13, wherein the timing circuitry is furtherconfigured to control timing of a window signal for activating thedetection circuit to monitor the current at the MEMS mirror, and whereinthe timing circuitry is configured to compare the measured length of thefirst activation pulse to a desired length by activating the windowsignal at the beginning of the first activation pulse and, based on thecurrent detected during the window signal activated at the beginning ofthe first activation pulse, determining the measured length is less thanthe desired length if the detected current is a first direction anddetermining the measured length is greater than the desired length ifthe detected current is a second direction.
 17. A method for controllingoperation of a resonance MEMS mirror, the method comprising: generatinga first set of activation pulses for operating the MEMS mirror;generating a window for current detection, the window overlapping withan end of an activation pulse; detecting current at the MEMS mirrorduring the window; and terminating the activation pulse in response todetecting a change in current direction during the window.
 18. Themethod of claim 17, wherein the MEMS mirror is an in-plane MEMS mirror.19. The method of claim 17, wherein the MEMS mirror is a staggered MEMSmirror.
 20. The method of claim 17, wherein generating the first set ofactivation pulses includes applying a first primary pulse to a firstportion of the MEMS mirror and applying a second primary pulse to asecond portion of the MEMS mirror.
 21. The method of claim 20, whereinthe first primary pulse commences when an angle of the MEMS mirror is atits greatest positive value and terminates at an angle of maximumcapacitance for the first portion of the MEMS mirror, and wherein thesecond primary pulse commences when the angle of the MEMS mirror is atits greatest negative value and terminates at an angle of maximumcapacitance for the second portion of the MEMS mirror.
 22. The method ofclaim 17, further comprising generating a second set of activationpulses for operating the MEMS mirror.
 23. The method of claim 22,wherein generating the second set of activation pulses includes applyinga first secondary pulse to a first portion of the MEMS mirror andapplying a second secondary pulse to a second portion of the MEMSmirror.
 24. The method of claim 23, wherein the first secondary pulsecommences when an angle of the MEMS mirror is at its greatest negativevalue and terminates at an angle of maximum capacitance for the firstportion of the MEMS mirror, and wherein the second secondary pulsecommences when the angle of the MEMS mirror is at its greatest positivevalue and terminates at an angle of maximum capacitance for the secondportion of the MEMS mirror.
 25. The method of claim 17, furthercomprising measuring a length of a first activation pulse and, based onthe measurement, adjusting a voltage of a subsequent activation pulse.26. The method of claim 25, further comprising: decreasing the voltageof the subsequent activation pulse if the length of the first activationpulse is less than a desired length; and increasing the voltage of thesubsequent activation pulse if the length of the first activation pulseis greater than the desired length.
 27. The method of claim 25, furthercomprising comparing the measured length of the first activation pulseto a desired length.
 28. The method of claim 27, wherein the comparisonincludes: activating the window at the beginning of the first activationpulse, detecting the current during the window activated at thebeginning of the first activation pulse, determining the measured lengthis less than the desired length if the detected current is a firstdirection, and determining the measured length is greater than thedesired length if the detected current is a second direction.
 29. Themethod of claim 27, wherein the length of the first activation pulse andthe desired length are measured in clock cycles.